Thiobacillus novellus (type strain) was grown chemolithoautrophically on thiosulfate in batch cultures under microaerophilic conditions. Under these conditions, T. novellus grew exponentially (mu = 0.05-0.06 h-1). The respiratory oxidation rates of tetrathionate, thiosulfate, elemental sulfur (S-degrees), and sulfite were measured respirometrically with an oxygen electrode, with exponentially growing cells. Cells growing on thiosulfate as the unique energy source retain thiosulfate-oxidizing activity, S-degrees-oxidizing activity (SOA), and very high sulfite-oxidizing activity, but lack respiratory tetrathionate-oxidizing activity. HQNO (50 mum), an inhibitor of the quinone-cytochrome b region, strongly inhibited the SOA (70%), moderately the sulfite-oxidizing activity (45%), and poorly the thiosulfate-oxidizing activity (15%), 1 mm KCN totally inhibited (>89%) all respiratory activities. This study confirms that in Thiobacillus novellus, as well as in other Thiobacilli, SOA is present in cells grown with thiosulfate as sole electron donor. SOA appears not to be an oxygenase; it is linked to the respiratory chain, and the electrons are probably released in the quinone-cytochrome b region.